This invention relates to monitoring combustion chambers in a gas turbine. In particular, the invention relates to a system and method for automatically identifying combustion chambers operating relatively hot or cold with respect to other chambers.
Dry low NOx (DLN) combustion systems are commonly used in modern industrial gas turbines. DLN combustion chambers receive premixed fuel and air at lean fuel-air (F/A) ratios to achieve low NOx emissions. These combustion systems are also required to comply with other emission constraints such as for carbon monoxide (CO) emissions. The level of emissions are strongly influenced by flame and combustion temperatures in the combustion chambers. Relatively lean A/F ratios and low combustion temperatures minimize NOx emissions. Because of their lean fuel flow, the chambers of DLN combustion systems are susceptible to burn outs if the A/F becomes too lean in one or more chambers. Individual variations of the F/A in the different chambers of a combustion system may cause some chambers to be more at risk for burn outs than other chambers with higher F/As.
Controlling emission levels is facilitated by operating each of the combustion chambers at uniform flame and combustion temperatures. Because of the non-linear relationships between flame and combustion temperature and emissions, relatively small changes in flame and combustion temperature can result in large changes in emissions. Because of the potentially large variations in emission levels, the combustion temperatures in each chamber of a combustion system should be uniform. To maintain uniform temperatures, the fuel to air (F/A) ratio in each combustion chamber should be substantially the same.
The F/A ratios may differ slightly from chamber to chamber in a combustion system. These chamber-to-chamber differences in the F/A ratios yield differences in the flame and combustion exhaust temperatures between the chambers and in turn cause the emission levels to differ from chamber to chamber. The differences in emission characteristics between the chambers are often relatively large and do not average out due to the non-linear transfer relationships between combustion temperatures and NOx and CO emissions. Accordingly, there has been a difficulty in maintaining uniform emission levels across all DLN combustion chambers in a combustion system of an industrial gas turbine.
Control systems regulate the flow of fuel to the combustion system to, in part, ensure that emissions comply with emission limits imposed on the gas turbine. Recent gas turbine fuel control systems enable the F/A ratio to individual combustion chambers to be dynamically adjusted. Individual control of the F/A ratio for each chamber is provided by fuel nozzle tuning valves and orifice plates that regulate fuel and air flow to a combustion chamber. The tuning valves and orifice plates may be dynamically adjusted during operation of the combustion system. A computerized fuel control system adjusts the tuning valves and/or orifice plates to each combustion chamber to, for example, achieve uniform flame and combustion temperatures. To make appropriate adjustments to the tuning valves and orifice plates, the control system requires data regarding the operation of the individual chambers.
There is a need for systems and methods to sense and collect data regarding the operating conditions in individual combustion chambers. This data may be applied to identify chambers operating relatively hot or cold as compared to other chambers. In the past, a correlation between combustion dynamic pressure frequencies and combustion temperature has been used to identify combustion chambers operating relatively hot or cold. Dynamic pressure sensors in combustion chambers detect combustion frequency tones in the chambers. Relatively low combustion frequencies are associated with cold chambers and high combustion frequencies are associated with hot chambers. Frequency tones in a range of 70 hertz (Hz) to 120 Hz are associated with combustion chambers operating at relatively cold combustion conditions, and tones in a range of the 120 Hz to 160 Hz region are associated with hot combustion chambers.
When the hot and cold combustion tones exist simultaneously in the same chamber it is difficult to determine whether the chamber is operating hot or cold. To determine whether a chamber is operating hot or cold when hot and cold tones frequency coexist, the amplitudes of the different combustion frequency tones have been used to determine whether a chamber is operating hot or cold. However, relying solely on combustion dynamic pressure frequencies and their amplitudes has not proven to be a robust method to identify hot and cold chambers.
The exhaust temperature sensed at the exit of the turbine has also been used to identify hot and cold combustion chambers See U.S. Pat. No. 6,460,346 and U.S. Patent Publication 2004/0148940. Thermocouples (TC) are arranged in an array at the turbine exhaust. These thermocouples provide a temperature profile of a cross-sectional area of the exhaust gas. It has been found that there should be twice as many TCs as combustion chambers to reliably use the exhaust temperature profile to detect hot and cold chambers. In addition, swirl charts are used to correlate the rotation of the combustion gasses to measured gas turbine parameters such as megawatt turbine output, compressor discharge pressure and firing temperature. These correlations may not be entirely accurate and often introduce uncertainty to ordering of combustion chambers on the basis of temperature.
There is a need for techniques to reliably identify combustion chambers operating hot or cold relative to the other chambers in the combustion system of a gas turbine. The system should accurately identify hot and cold chambers and rank the chambers based on their combustion temperatures.